1. Field of the Invention
This invention relates to a sheet processing apparatus for processing sheets, and to an image forming apparatus, more particularly to a technique of improving the alignment of sheets temporarily placed on a sheet tray.
2. Description of the Related Art
Some of the image forming apparatuses for use in copiers, laser-beam printers, facsimiles and composite apparatuses, each comprising these, have a sheet processing apparatus that performs a sheet processing such as binding sheets each having an image formed on it.
In such an image forming apparatus, the sheet processing apparatus will make an untidy sheet bundle if the sheets are not aligned on the sheet tray. The untidy sheet bundle must be unbundled and bundled again. Therefore, it is important to align the sheets well in the sheet processing apparatus.
It is recently demanded that the sheet processing apparatus should hold many sheets and process them at high speed. An apparatus that may meet this demand is disclosed in Japanese Patent No. 4,298,360 (corresponding to U.S. Pat. No. 7,192,020 B2 and to Chinese Patent 100335388 C, hereinafter referred to as “patent documents”). This apparatus, shown in FIG. 22 attached thereto, is a sheet processing apparatus BO to be incorporated into image forming apparatuses. In the sheet processing apparatus BO, a relatively large number of sheets are placed on the sheet tray T, bound together in the binding unit TSP and delivered, in the form of a bundle, by the delivery rollers R onto the accumulating tray AT. The sheet processing apparatus comprises a saddle-binding tray NT and a saddle-binding unit NSP. The saddle-binding tray NT branches from the conveyance path at the inlet port of the apparatus. The saddle-binding unit NSP binds sheets at the middle part of the saddle-binding tray NT.
In this apparatus, while the binding unit TSP is binding the sheets placed on the sheet tray T, the following sheets are kept waiting at a standby tray BT so that they may be processed in a large number and at high speed. The standby tray BT (generally called “buffer tray”) is designed to keep one to three sheets waiting. While the sheets are so kept waiting, the sheets on the sheet tray T are bound together, forming a bundle.
As shown in FIG. 23A, the sheet bundle BP and the bundle of the following sheets are conveyed from the sheet tray T and the standby tray BT, respectively, with a prescribed offset, are nipped at delivery rollers R (i.e., upper delivery roller R1 and lower delivery roller R2), and are delivered onto the accumulating tray AT. The sheet bundle BP is thereby fed from the delivery rollers R and mounted on the accumulating tray AT.
Meanwhile, the following sheets (WP1 to WP3) are nipped by the delivery rollers R. At this time, the delivery rollers R are stopped for some time and then rotated in the reverse direction. The following sheets are thereby switched back above the sheet tray T and then placed on the sheet tray T. This method of delivering the sheets is generally called “simultaneous bundle delivering”, and enhances the speed of delivering the following sheets from the standby tray BT. Ultimately, the apparatus can operate at high speed.
In the apparatus disclosed in the above-identified patent document, the last following sheet WP3 is set nearer to the front end of the sheet bundle BP than the other following sheets WP1 and WP2 by distance wp11. This is because the following sheets may be conveyed in wrong order as the delivery rollers R rotate, and such wrong-order conveyance must be prevented. (See FIG. 47 of the above-identified patent document.)